The present invention relates to an optical head and an image forming apparatus incorporating the same.
Generally, a toner image forming device of an electrophotographic system comprises: a photosensitive body as an image carrier having a photosensitive layer on an outer peripheral surface thereof; a charger that uniformly charges the outer peripheral surface of the photosensitive body with electricity; an exposer for selectively exposing the outer peripheral surface, which is uniformly charged with electricity by the charger, to form an electrostatic latent image; and a developing device that applies toner serving as a developer to the electrostatic latent image formed by the exposer to make the latent image a visible image (toner image).
An image forming apparatus of a tandem type that forms a color image includes one of an intermediate transfer belt type, in which the toner image forming device described above are arranged in plural (for example, four) for an intermediate transfer belt, toner images formed on the photosensitive body by the single color toner image forming device are sequentially transferred to the intermediate transfer belt, and toner images of plural colors (for example, yellow, cyan, magenta, black) are overlapped together on the intermediate transfer belt to obtain a full color image on the intermediate transfer belt.
In the image forming apparatus of a tandem type, an optical head provided with a LED array or an organic EL element array is sometimes used as exposer. For example, Japanese Patent Publication No. 11-138899A discloses a light emitting diode array, in which light emitting diodes are mounted on a single chip.
An explanation will be given to an example of an optical head, in which an organic EL element is used for a light source and an image is formed on an image carrier by a rod lens array optical system. FIG. 6 shows such an optical head (image writer) 23 in a direction that the image carrier moves. A rod lens array 65 in which gradient index rod lenses 81 are arrayed is mounted to an opaque housing 60. An photo emitter array 61 is mounted so as to face a back face of the rod lens array 65 in the housing 60.
An opaque cover 66 shields the photo emitter array 61 in the housing from a back face side of the housing 60. The reference numeral 63 denotes a photo emitter (organic EL element), and 64 a cover glass. A plate spring 67 pushes the cover 66 against the back face of the housing 60 to close an interior of the housing 60 in a light-tight manner. The housing 60 covers a periphery of a glass substrate 62 but opens a side thereof facing an image carrier 20. Thus light outgoing from the photo emitter 63 is projected onto the image carrier 20 through the rod lens 81.
In the optical head shown in FIG. 6, a ghost light spot 93 is sometimes formed on the image carrier 20. The reason for this will be described below. When the photo emitter 63 is to be formed, an anode and a cathode are used. Since the cathode is formed from aluminum or the like, it serves as a mirror to reflect stray light in the grass substrate 62. Therefore, reflected light 95 transmits through the rod lens 81 whereby the ghost light spot 93 is formed on the image carrier 20.
FIG. 7 shows the image carrier 20 by seeing through the rod lens array 65. In this figure, the reference numerals 91, 92 denote exposure spots formed on the image carrier 20 by the photo emitter 63, CL denotes a center line of rod lens array 65, and 93, 94 denote ghost light spots. The ghost light spots include a large ghost light spot 93 formed to be concentric with the respective rod lenses 81, and a small ghost light spot 94 formed in the vicinity of the border between adjacent rod lenses 81. Such ghost light spots 93, 94 are formed on the image carrier 20 whereby there is caused a problem that image unevenness is formed to deteriorate the quality of image formation.